1. Field of the Invention
This invention pertains generally to name services for computer networks, and more particularly to name services that are enhanced to provide automatic revision features.
2. Description of the Background Art
With the advent and growth of network computing, name services have become increasingly important. Name services store and provide information that workstations, applications, and users need to communicate across the computer network. These services eliminate the need for each workstation or user to maintain its own copy of this information.
For example, a given workstation in a computer network requires the network address of other workstations in order to send messages. If there were no name service in the network, the workstation would have to store a file containing the addresses of all workstations, including itself. This file would also be stored on every other workstation in the network. In addition, each workstation would store duplicate files of other network-related information, such as mail information, security information, information about network services, and so on. As networks grow larger, the increasing amount of network information taxes the capacity of the workstations. Furthermore when the network changes, for example by adding or removing a workstation, the list of addresses and other network information must be updated on each workstation. This updating task becomes difficult in large networks.
Name services address this problem by providing a centralized source of network information that is accessible by the individual workstations. The information is stored on a server, which is connected to all workstations in the network. Generally the term "server" means a discrete host computer in a network, and it provides services to other computers, termed "clients". In short, the name service is a program running on a server host containing files of network-related information, such as a mapping of host names to network IP ("Internet Protocol") addresses of all computers in the network. The name service provides such mapping information to other computers and workstations upon request.
It should be noted that the term "server" conventionally has a second meaning. Typically in a distributed "client-server" computing environment, a "server" may be a program running on a particular host which provides network-related services, and this host may contain other server programs, and also be a client of still other servers. The term "server" is also used in this sense in the following description, as will be apparent from the context.
The name service provides several features to the computer network. First, it reduces the amount of overall space that must be devoted to storing network information. Secondly, it reduces the amount of effort required of a system administrator to update the network information to reflect changes in the network. A system administrator periodically updates the files in the name service on the server's host only, instead of all of the individual workstations. In addition, the name service provides greater flexibility and simplicity in handling the information itself. For example, the IP addresses in a network are numerical labels which are generally partly assigned by a system administrator and partly determined by the network topology. The name service allows one to translate a textual name of a host into its IP address. This mapping can be modified in software to reflect changes in the organization of the network components.
The name service stores the network address information in a file having the form of a map. The map is essentially a bi-column table that establishes a correspondence between textual names and the numerical addresses of the network components. The map can be used to search for the numerical address corresponding to a given name, or the names corresponding to a given numerical address. Similar maps may be used to store other network-related information.
In practice, the name service is normally replicated for purposes of reliability. The name service files are created in principal servers, or "master" servers, and are duplicated in backup servers, or "slave" servers. A client may utilize the name service by accessing any one of these servers.
The name service described above must be updated manually by an administrator at periodic intervals to keep track of changes in the network information. At each update session the administrator enters new information into the various maps on the master server. The administrator then propagates the modified maps to all of the slave servers; or the system may propagate the maps periodically on its own. This updating technique is satisfactory for networks that are not too large. However modern networks may have hundreds of servers supporting thousands of clients, and manual updating can be a time-consuming and arduous process. Furthermore in larger networks the network information changes occur more frequently. In short, the manual updating process for the name service encounters serious problems in a large network.
Automated name services have been designed to overcome the updating problem in large networks. These services are referred to as "dynamic" name services, to distinguish them from the "static" name services described above. Dynamic name services provide for incremental updates, in which programs directly update maps on the master server and in which only the changes in the name service data are sent to the slave servers from the master server, and these changes are automatically propagated. These dynamic name services have a different design from static name services. Thus, when a network outgrows the static name service, conversion to a dynamic service is often a major undertaking, and many large networks still utilize a static name service.
As discussed above, a name service can map a host name to a host IP address. A name service can also provide other mappings. For example, it can provide a client with access to a network-wide service, by mapping a textual name to a program number associated with that service (i.e. an integer). If the required service is available only on one host instead of all hosts, then the mapping becomes more complicated--mapping a textual name to a pair which consists of a host name and a program number. These are all examples of mappings that need updating only occasionally (for example, when a new host or service is added); the lack of automatic updating is not a serious problem.
If a particular host provides multiple instances of a service that differ in small ways (such as service instances that each provide access to one print server), then each instance may require its own program number. Program numbers are assigned from a fixed range of integers, divided into a static pool and a dynamic pool. A central authority, which is generally a person and may or may not be a system administrator, assigns program numbers from the static pool for programs to use permanently. A program can temporarily borrow program numbers from the dynamic pool. When a program terminates, its borrowed numbers are automatically returned to the dynamic pool.
Because the static range is finite and the number of service instances on a host is unknown but potentially large, the static pool solution has drawbacks. Such a solution would consume too many program numbers and would still have to use the dynamic pool if the number of service instances exceeded the number of program numbers assigned in a static manner.
The program numbers must come from the dynamic pool. Because it is impossible to assure that a service instance (e.g. a print server) has the same program number each time it starts, the name service must provide access to a service instance whose program number is continually changing. This changing program number requirement is easily satisfied in environments with a dynamic name service, but not in environments with a static name service. In short, it is desirable to provide dynamic behavior to a static name service.